Surface mounted current sense resistors have been available for the electronic market for many years. Their construction typically includes a flat strip of a resistive material that is coupled between high conductivity metal terminals forming the main terminals of the device. A pair of voltage sense terminals can be formed in the main terminals thereby creating a four terminal device. The main terminals carry the majority of the current through the device. The voltage sense terminals produce a voltage that is proportional to the current passing through the device. Such devices provide a mechanism to monitor the current passing through a given circuit using conventional voltage sensing techniques. The actual current passing through the device can be determined based on the sensed voltage and the resistance value of the device as dictated by ohms law. An ideal device would have a Temperature Coefficient of Resistance (TCR) that is close to zero. However, most devices have a non-zero TCR that can lead to inaccurate voltage readings at the voltage sense terminals particularly when the temperature of the device varies.
In low ohmic current sense resistors and high current shunts, the resistive element length is short while the length of the resistor is a standard length, or in the case of high current shunts long because of the application. The long resistor length and short resistive element length causes a significant amount of copper termination metal to be in the current path. Copper has a TCR of 3900 ppm/° C. while the resistive material is typically less than 100 ppm/° C. The added copper in the current path drives the overall TCR of the resistor to values that can be in the 800 ppm/° C. range or greater, versus a desired TCR of less than 100 ppm/° C.
As noted above, typical current sense resistors have four terminals, two main terminals and two voltage sense terminals, separated by two slots. The length of two slots is manipulated to adjust TCR. See U.S. Pat. No. 5,999,085 (Szwarc). This method does not lend itself to conventional resistor calibration equipment such as a laser or other cutting techniques that are typically used to reduce the width of the resistive element to increase the resistor's resistance value.
What is needed is an improved configuration and method of making a current sense resistor with TCR compensation or adjustment. It would also be desirable to provide an improved resistor configuration and method that simplifies TCR adjustment of current sense resistor during the manufacturing process. One or more of these aspects will become apparent from the specification and claims that follow.